In the manufacture of semiconductor devices, it is frequently necessary to form a dielectric layer either as a passivation layer to permanently protect the surface of the substrate or as a mask during such procedures as etching, solid state diffusion, or ion-implantation. One method by which an oxide layer, such as silicon dioxide (SiO.sub.2), may be formed is a plasma-enhanced chemical vapor deposition process as described by James A. Amick, G. L. Schnable, and J. L. Vossen, in the publication entitled, "Deposition Techniques for Dielectric Films on Semiconductor Devices," in the Journal of Vacuum Science Technology, Vol. 14, No. 5, September-October 1977, pp. 1053-1063. In such a plasma-enhanced process, the vapor phase reactants, such as silane (SiH.sub.4) and nitrous oxide (N.sub.2 O), are subjected to a radio frequency discharge, which creates an ionized plasma of the reactant gases. These ionized reactants then interact to form the desired reaction product. However, as a result of the exposure of the reactant gases to the radio frequency discharge, numerous extraneous ionized and neutral particles, as well as high energy radiation with wavelengths as low as 500 angstroms (A) and even extending into the x-ray region, are produced and bombard the surface of the substrate on which the oxide is being formed. If the substrate comprises a sensitive device type, such as a charge coupled device or a device formed of certain compound semiconductors (e.g., InSb, HgCdTe, or GaAs), the above-described charged particles and unwanted radiation frequently impart damage to these sensitive devices. For example, the deposited oxide layer may incorporate charges or dangling bonds, which create high surface state densities (N.sub.ss) at the semiconductor device/oxide layer interface and which will trap charges when a voltage is applied to the device, thereby preventing optimum device performance. In addition, a plasma enhanced deposition process has the disadvantage that plasma-induced heating of the substrate occurs as a result of selective absorption of the radio frequency energy by the substrate, and this heating causes uncertainty in the temperature of the substrate, which prevents optimization of the characteristics of the deposited oxide layer.
Other methods by which oxide layers may be formed use a non-reactive or a reactive sputtering technique. By a non-reactive sputtering technique, as described, for example, by Amick et al as referenced above, a disk of the selected oxide material, such as SiO.sub.2, is bombarded with argon ions, which cause the SiO.sub.2 to vaporize, and the vaporized SiO.sub.2 subsequently deposits on the selected substrate. By a reactive sputtering technique, as described, for example, by Amick et al as referenced above, a disk of silicon is bombarded with oxygen ions, which causes vaporization of the silicon, and the vaporized silicon and oxygen ions then react to produce the desired SiO.sub.2. However, these sputtering techniques are similar to the above-described plasma processes in that they frequently impart damage to sensitive devices due to charge bombardment or radiation bombardment of the device. In addition, the films produced by sputtering techniques are often granular, not dense, and not specular (i.e., having good light reflecting properties).
Both the sputtering technique and the plasma-enhanced method for chemical vapor deposition may be used to deposit a dielectric layer which incorporates a selected dopant material. In the former case, an appropriately doped target could be bombarded by selected ions. In the latter case, a dopant-containing material is added to the reactant gases which are then ionized. However, both the above sputtering technique and the above plasma-enhanced method suffer from the difficulties discussed above, particularly that of imparting damage due to charge bombardment or radiation bombardment and plasma-induced heating of the substrate.
Still another known method by which oxide layers may be formed involves thermal processes. In order to form SiO.sub.2, for example, by a thermal process, silane is brought into contact with oxygen at a low temperature and a spontaneous reaction occurs, forming SiO.sub.2. The films formed by thermal processes, however, are usually granular, do not necessarily have good adhesion, and tend to incorporate traps.
It is the alleviation of the prior art problem of imparting damage to sensitive devices due to charge bombardment or radiation bombardment during the formation of an oxide layer thereon to which the present invention is directed.